SPN 605 FMI 31: Meaning and Fix
SPN 605 FMI 31 indicates the Refrigerant High Pressure Switch reports a continuous ‘pressure too high’ condition (binary 01b) to the ECM via the Shutdown PG. This fault commonly appears after a failed DPF regeneration event where elevated underhood temperatures cause refrigerant pressure spikes. The ECM then disengages the compressor clutch to protect the system, often leading to a loss of cab cooling and an engine torque derate of up to 25% on MAN and Mercedes-Benz platforms.
Common Symptoms
- No AC Cooling: Compressor clutch remains disengaged; cab interior temperature rises rapidly during operation.
- Engine Torque Derate: ECM reduces engine power by up to 25% to lower heat load on the refrigerant circuit.
- High Side Pressure Spike: Observed refrigerant pressure exceeds 28 bar on MAN TGS or Mercedes Actros HVAC gauges.
- Fault Lamp Illumination: Amber warning lamp and AC fault indicator activate on the dashboard within one drive cycle.
Probable Causes
- Stuck Closed Switch: Refrigerant high pressure switch diaphragm mechanically seized in the closed position.
- Overcharged Refrigerant: Excess R134a or R1234yf charge causing static pressures above 20 bar at ambient 25°C.
- Condenser Airflow Blockage: Debris or damaged fan clutch reduces heat rejection, raising high side pressure above threshold.
- Wiring Short to Battery: Signal wire chafed against chassis ground or +12V supply, forcing ECM input high.
Advanced Technical Analysis
The ECM monitors the high pressure switch via a digital input with an internal pull-up resistor. When the switch is open (normal, binary 00b), the input reads low. When closed (high pressure, binary 01b), the input reads high. FMI 31 is set when the ECM detects the high state continuously for more than 5 seconds without interruption, indicating a persistent condition rather than a transient spike.
Electrical analysis reveals that the ECM applies a debouncing timer of 200 ms to filter mechanical chatter. If the signal remains high after debouncing, the fault is logged. A short to battery on the signal line (pin 14 of the Shutdown PG) can mimic a closed switch. Using a breakout box, technicians should verify 0V with switch open and battery voltage with switch closed at the ECM connector.
Upon activation, the ECM immediately disengages the compressor clutch via a PWM output and sets a torque derate curve that reduces maximum fuel injection quantity. On Deutz TCD 7.8 engines, this derate is linear from 100% to 75% over 30 seconds. The AC request from the HVAC controller is ignored until the fault is cleared by cycling ignition off for 60 seconds.
Long-term prevention involves checking condenser airflow and refrigerant charge. Real-world workshop data from Bosch shows that 40% of SPN 605 FMI 31 cases are caused by overcharging after compressor replacement. Using a recovery machine to evacuate and recharge to exact factory specifications (e.g., 1150g ± 50g for MAN TGS) eliminates recurrence. Always inspect the switch for corrosion at the Schrader valve interface.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check condenser for debris and fan clutch engagement. Verify refrigerant sight glass for bubbles.
- Switch Test: Disconnect switch, measure resistance: open circuit normal, <0.5 ohms when closed. Replace if stuck.
- Wiring Integrity: Check signal wire for shorts to ground or battery using multimeter. Repair chafed insulation.
- Refrigerant Recovery: Recover charge, weigh, compare to spec. Evacuate and recharge to exact OEM mass. Clear fault.